Just went through this myself
I just went through this myself. I went with 10' oars. I bought a 16' Avon earlier this year and needed to pick up oars. The people at Down River Equipment were awesome about helping me figure it out even though I didn't have all of the measurements. They talked about the same things some other people have mentioned (how high you sit above the frame and interior width). They recommended having about 1/3 inside the locks and 2/3 outside the locks.